This invention relates to a process of producing polycarbonates by reacting a monomeric carbonate component with at least one diphenol or dialcohol in the presence of a transesterification catalyst, wherein the molten components are mixed with the transesterification catalyst by stirring and a transesterification product is produced, which is polycondensated.
In known processes for the continuous production of polycarbonates, up to 6 stirred tank reactors with vertical agitator shaft are used before the final reactor. As regards conversion and dwell time spectrum of the melt, these stirred tanks have considerable disadvantages over reactors through which the melt flows horizontally, for example. For the same conversion, a stirred tank requires a reaction volume which is 4 to 100 times as large, depending on the reaction rate, so that longer dwell times are obtained. As regards the dwell time spectrum of the melt, the stirred tank also has considerable disadvantages, because the concentration of the melt supplied is immediately diluted to the concentration in the stirred tank, which also is the outlet concentration. As compared to the reactor with horizontal plug flow and almost the same local dwell time of the reactants, this leads to a wider distribution of the dwell times. In the stirred tank, some particles quickly reach the reactor outlet together with the flow, while others dwell for a longer period together with the circulating flow, which is a reason for the width of the dwell time distribution. It should furthermore be considered that with very large reaction volumes the efficiency of the stirrer and the installations impeding the circulating flow likewise have a substantial influence on the dwell times of the melt in the reaction space. In the case of polymerization reactions, this behavior is detrimental, as long dwell times of a part of the reaction mass promote undesired side reactions and the distribution of the macromolecule chains is widened in an undesired way. It is known that the side reactions lead to more or less strong discolorations in the product.
It is the object underlying the invention to produce polycarbonates with a rather narrow distribution of the molecular weights and little side-chain branching. Furthermore, the product should rather not have any black particles, only a negligeably small amount of yellow coloring and only a low content of gel.